A biology lesson at Berkeley High School has turned into a mission for 11 students who will spend their last summer of high school helping build an addition to a schoolhouse in the East African country of Tanzania.

Inspired by a teacher's monthlong course on the global effect of communicable diseases such as AIDS, the teenagers spent the past three months raising $20,000 to fund their trip.

The students took off from San Francisco International Airport on Friday morning with their biology teacher Mary Murphy and another Berkeley High teacher, Leah Katz. They will stop in Toronto and London before landing in Nairobi, Kenya, this afternoon. Late this evening, San Francisco time, they will take a nine-hour van ride to Shirati, Tanzania.

For 32 days, they will live in Shirati, a small village on the coast of Lake Victoria taxed by the number of AIDS patients and children orphaned by the epidemic.

It's an opportunity to see the AIDS crisis as more than a bunch of statistics, said 17-year-old Casey Laird.

"You hear that 2 million people (die from) tuberculosis, and you think, 'Oh, that sucks.' It doesn't hang in your mind at all. It's just a big number," Casey said. "I want to sit there for a while and become a part of the community, see them as friends and families with lives. It will open my mind to what the numbers mean."

Tanzania claims the world's fifth-largest number of adults living with AIDS -- about 1.6 million, or 9 percent of the country's adult population, according to a 2003 report by the World Health Organization.

Nearly 1 million children in the country have lost one or both of their parents die to the disease, health experts say.

The Berkeley students will live in Shirati in a five-bedroom house owned by Christine Nyada-Chacha, a former Berkeley High School teacher originally from the village and now living in El Sobrante.

The house has no running water and an unreliable electrical system. Clothes must be washed by hand, and the toilet must be manually filled with water each time it is used.

The new addition to the schoolhouse is a community effort, and the structure will be built with bricks made by hand.

"It will be hard to get used to. It will be completely different from anything I've seen," said 16-year-old Sarah True. But she added, when Murphy first told her about the trip, "Right then and there, I was like, 'Yes. We have to do this.' "

True and the 10 other students are in Berkeley High's Communication Arts and Sciences school, a program with its own teachers and 300 students.

In March, Murphy, Katz and the students' families began a campaign to raise money for the trip. They earned $6,000 cooking a traditional Tanzanian dinner and auctioning donated East African art and gift certificates.

One student baked cookies and sold them for 25 cents.

When they weren't fundraising, the students solicited school and medical supplies for the village. They collected more than 650 pounds worth -- enough to fill 13 suitcases -- from latex gloves and bandages to crayons and soccer uniforms.

"It's been a lot of work," said 16-year-old Max Perel-Slater. "People have given up a lot to go on this trip."

Since his freshman year, Max had planned to spend this summer at a selective crew training camp in Sacramento.

"I think this is much more important," he said. "(The camp) was something I would do for myself, but this is something I'm doing for my community."